Conventional motors are typically comprised of a rotational portion (a “rotor”), a stationary portion (a “stator”) to which the rotor is rotationally coupled, and circuitry that causes the rotor to rotate in a predetermined manner.
The speed of a DC motor is governed, in part, by the voltage applied to its field windings. Consequently, the voltage appearing across the field windings may significantly affect whether the motor operates within the design specifications of the application in which the motor is employed. On the other hand, it may be desirable to provide for operation of the motor under a large range of available voltages.
Therefore, regulator circuits, or DC-to-DC converters, are provided in order to supply a specified voltage across the motor field windings at any particular time. Various regulator circuits are employed in the art, typically characterized by a significant inductive reactance which allows energy to be stored during a portion of each of a series of pulses imposed on the input supply voltage.
One example of a regulator circuit is a buck regulator. FIG. 1 depicts a typical buck regulator circuit 8. In the buck regulator 8, a controller 10, which can be an integrated circuit, provides a periodic switching voltage to a pass switch 12, represented in FIG. 1, for example, by a transistor, thereby allowing input voltage Vin to be applied, with some periodic functional form, across a rectifier, or diode 14. Current flows in the circuit containing an inductor 16, a capacitor 18, and the diode 14 during complementary portions of the periodic function to those of current flow through the pass switch 12 such that energy is effectively stored in the inductor 16 during those portions of the cycle when power is not being supplied directly by current through the pass switch. Thus, high efficiency is advantageously obtained, and the power delivered by applying output voltage Vout across the motor field windings, as load, is only weakly dependent on input voltage Vin. The controller 10 adjusts the switching duty cycle of the pass switch 12 in order to maintain a specified output voltage.
The presence of the buck regulator 8, however, increases the volumetric space requirements within a motor housing in the design of a motor for a specified application. Moreover, circuit inefficiencies lead to regulator circuit heating and increased heat dissipation requirements. In particular, the requirement of a large inductor, typically characterized by inductances on the order of 100 μH, and current capacity on the same order as that of the motor field windings, impose significant design costs in both “real estate” and manufacturing cost.